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Secondary Radar Transponder
Testing Using the 8990B Peak
Power Analyzer
Application Note
Introduction After a brief review of radar systems and the role of transponders, this applica-
tion note provides examples of how to effectively test transponders in order to
validate their performance and function. Testing is performed using a transpon-
der test set and an Agilent peak performance analyzer (PPA). The measurement
examples provided cover interrogation and reply transmit power and pulse
profiling, double pulse spacing, and reply delay timing measurement.
Secondary Radar Secondary radar originated from the identification friend or foe (IFF) radar signal
system used during World War II and complements the limitations of primary
Background radar systems.
Primary radar works by passively reflecting a radar signal off of the target's
reflection or surfaces (called echoes). A limitation of primary radar is that it has
difficulty detecting non-metal or composite-based aircraft parts. Another weak-
ness is weather-related. In heavy rain, reflected signals are prone to attenuation,
decreasing detection accuracy.
Secondary radar works by transmitting and receiving high-frequency modulated
pulses, also called interrogation and reply signals. Figure 1 illustrates the operat-
ing principle of secondary radar systems. It begins when the ground station
sends interrogation signals to the airborne aircraft. The plane's on board tran-
sponder responds to the interrogation signals by transmitting back reply signals.
Modern secondary radar systems are used in both civilian and military aviation
operations. The civilian's secondary radar system is called secondary surveil-
lance radar (SSR) and it is primarily used for air traffic control such as in the
Air Traffic Control Radar Beacon System (ATCRBS) and the Traffic Collision
Avoidance System (TCAS). SSR operates in different modes known by letter
designators such as Modes A, B, C, D, and S. Rather than alphabetic modes, the
military IFF uses numerical modes 1 through 5. The military and civilian modes
operate differently but modes 3 and A are similar and mode 5 is an encrypted
version of mode S.
Interrogation from
ground station
Reply from transponder
On-board transponder
Ground
station
Figure 1. Illustration of the secondary radar operating principle
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What is a Transponder As mentioned previously, transponders are an important part of the secondary
radar system. Usually mounted on the under surface of the aircraft's fuselage,
and Its Function? the transponder is basically a transmitter and receiver. As shown in the timing
diagram in Figure 2, during operation the transponder receives interrogation
pulse pairs from the ground station and decodes the requested enquiries. After
a certain delay duration, the transponder then responds with a different series of
pulses that contain the information requested by the interrogation transmission.
The communication exchanges can include information such as the aircraft
identifier, altitude, and bearings. The interrogation and reply pulses use different
frequencies, depending on the mode of operation.
P1 P3
Interrogation
pulse to
transponder
Reply delay
Double pulse spacing
F1 F2
Reply pulse
from
transponder
Reply pulse spacing
Figure 2. Transponder interrogation and reply pulse pairs timing diagram. (Note: When
radar is in use, a P2 interrogation pulse is transmitted and ignored. F1 and F2 refer to
framing reply pulses.)
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Testing and Validating Federal aviation safety standards, such as those defined by the US Federal
Aviation Administration, require transponders to undergo periodic maintenance
the Transponder's and calibration. This precaution ensures that the transponder is decoding inter-
Performance and rogation pluses correctly and subsequently replying with correct pulses. The
maintenance also includes performance checks that ensure the transponder
Functions transmit/receive functions conform to specifications. Transponder calibrations
are typically done using a transponder test sets and the Agilent 8990B peak
power analyzer (PPA).
Ultimately these maintenance tasks optimize efficiency and minimize the
potential for transponder failure during operation. They also ensure compliance
with aviation safety standards. A malfunctioning transponder can result in
a catastrophic event. From the military operation perspective, a transponder
failure such as an incorrect reply can ultimately mean the difference between
life or death.
The following sections demonstrate how the Agilent 8990 PPA is used to
perform transponder and transponder test set maintenance and validation. The
measurement examples featured are interrogation and reply transmit power and
pulse profiling, double pulse spacing, and reply delay timing measurement.
Example 1: IFF transponder This example explains how to measure the IFF transponder's simple reply pulse.
reply tests The objective of the test is to ensure that the transponder generates the correct
reply pulses using the correct reply delay. Figure 3 shows the measurement
setup.
Agilent 8990B PPA
Agilent MXG Attenuator
Interrogation pulse
Coupler
N1923A
Reply pulse
IFF transponder
under test
RF IN/OUT
Attenuator N1923A
Coupler
Figure 3. Transponder interrogation and reply pulse measurement
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Example 1: IFF transponder 1. Connect the transponder under test to a signal generator using directional
reply tests (continued) couplers.
2. Using pulse building software such as Agilent N7620A Signal Studio,
construct interrogation pulses according to the operating modes as shown in
Table 1. In this application example, the transponder is set to operate using
Mode 1 for the interrogation and reply test.
Table 1. Example of the IFF interrogation double pulse specifications
Interrogation frequency 1030